On 23/02/11 03:27, Robert Haas wrote:
> On Tue, Feb 22, 2011 at 6:26 AM, Greg Stark<gsstark(at)mit(dot)edu> wrote:
>> Actually it's not waiting for the LockBuffer LWLock. it's waiting
>> until your query unpins the buffer it wants. Vacuum tries to get an
>> exclusive lock on the buffer, if it gets it then it checks if anyone
>> is using that buffer. If someone is then it unlocks the buffer and
>> waits until nobody has it pinned.
> How bad it would be if we made LockBufferForCleanup() not wait? If we
> can't obtain the buffer cleanup lock immediately, we just skip that
> page and continue on. That would prevent us from updating
> relfrozenxid, I guess, but we already can't do that if there are any
> bits set in the visibility map. It could also leave some bloat in
> the table, but probably not much (he says hopefully).
>
Seems like a good suggestion, and may leave less bloat than having the
vacuum hung for potentially quite some time.
Mark